Breach should take down everything as long as you can get it off the ground. Improved Invisibility and spells like Minor Spell Deflection (but not Minor Globe of Invulnerability) will prevent you from hitting them with it, as will Spell Shield. However, I don't think any mages in SoD use those buffs, at least at Boareskyr Bridge.

Dispel Magic only has a chance of working, depending on the caster's level and the target's level. A level 8 player-controlled mage would have a 50% chance of removing a level 8 enemy mage's buffs.

Minor Globe of Invulnerability, a common buff for enemy mages, cannot be dispelled. Secret Word and Spell Thrust can both take it down 100% of the time. I'm guessing that's why Breach and Dispel Magic don't seem to do anything; the visual effect of Minor Globe of Invulnerability is the easiest to see, and if it doesn't vanish, it looks like the mages' defenses remain untouched.